Our sales mix can be impacted by severe or unusual weather over a short-term period. Over the long-term, we believe the impact of the weather on our sales mix is not significant.
The two statistics we believe have the most positive correlation to our market growth over the long-term are miles driven and the number of seven year old or older vehicles on the road. While over the long-term we have seen a positive correlation between our net sales and the number of miles driven, we have also seen time frames of minimal correlation in sales performance and miles driven. During the periods of minimal correlation between net sales and miles driven, we believe net sales have been positively impacted by other factors, including the number of seven year old or older vehicles on the road. The average age of the United States light vehicle fleet continues to trend in our industry’s favor. According to the latest data provided by the Auto Care Association as of January 1, 2018, for the seventh consecutive year, the average age of vehicles on the road has exceeded 11 years. Since the beginning of 2018 and through March 2019 (latest publicly available information), miles driven have increased by 1.8%.
Twelve Weeks Ended May 4, 2019
Compared with Twelve Weeks Ended May 5, 2018
Net sales for the twelve weeks ended May 4, 2019 increased $122.9 million to $2.783 billion, or 4.6%, over net sales of $2.660 billion for the comparable prior year period. The increase in net sales was partially offset by the sale of two businesses in the prior year. Total auto parts sales increased by 4.7%, primarily driven by an increase in domestic same store sales of 3.9% and net sales of $44.7 million from new domestic AutoZone stores, partially offset by the impact of the sale of a business completed in the prior year. Domestic commercial sales increased $79.6 million, or 14.9%, over the comparable prior year period.
Gross profit for the twelve weeks ended May 4, 2019 was $1.492 billion, or 53.6% of net sales, compared with $1.423 billion, or 53.5% of net sales, during the comparable prior year period. The increase in gross margin was attributable to the impact of the sale of two businesses completed in the prior year (29 basis points), partially offset by lower merchandise margins driven primarily by a shift in mix.
Operating, selling, general and administrative expenses for the twelve weeks ended May 4, 2019 were $944.5 million, or 33.9% of net sales, compared with $877.2 million, or 33.0% of net sales during the comparable prior year period. Deleverage was primarily driven by increased domestic store payroll (69 basis points) and increased incentive compensation, partially offset by the impairment related to the sale of two businesses in the prior year.
Net interest expense for the twelve weeks ended May 4, 2019 was $43.2 million compared with $42.0 million during the comparable prior year period. The increase was primarily due to an increase in average borrowing levels over the comparable prior year period. Average borrowings for the twelve weeks ended May 4, 2019 were $5.191 billion, compared with $5.071 billion for the comparable prior year period. Weighted average borrowing rates were 3.2% for the twelve weeks ended May 4, 2019 and the comparable prior year period.
Our effective income tax rate was 19.5% of pretax income for the twelve weeks ended May 4, 2019, and 27.2% for the comparable prior year period. The decrease in the tax rate was primarily attributable to additional tax benefits from option exercises recognized in the current period versus the twelve weeks ended May 5, 2018 (see “Note O – Income Taxes” in the Notes to the Condensed Consolidated Financial Statements).
Net income for the twelve week period ended May 4, 2019 increased by $39.2 million to $405.9 million due to the factors set forth above, and diluted earnings per share increased by 19.2% to $15.99 from $13.42 in the comparable prior year period. The impact on current quarter diluted earnings per share from stock repurchases since the end of the comparable prior year period was an increase of $0.98.
Thirty-Six Weeks Ended May 4, 2019
Compared withThirty-Six Weeks Ended May 5, 2018
Net sales for thethirty-six weeks ended May 4, 2019 increased $213.0 million to $7.875 billion, or 2.8%, over net sales of $7.662 billion for the comparable prior year period. The increase in net sales was partially offset by the sale of two businesses in the prior year. Total auto parts sales increased by 3.7%, primarily driven by an increase in domestic same store sales of 3.1% and net sales of $129.7 million from new domestic AutoZone stores, partially offset by the impact of the sale of a business completed in the prior year. Domestic commercial sales increased $193.9 million, or 13.1%, over the comparable prior year period.
Gross profit for thethirty-six weeks ended May 4, 2019 was $4.235 billion, or 53.8% of net sales, compared with $4.066 billion, or 53.1% of net sales, during the comparable prior year period. The increase in gross margin was primarily attributable to the impact of the sale of two businesses completed in the prior year (54 basis points) and higher merchandise margins.
Operating, selling, general and administrative expenses for thethirty-six weeks ended May 4, 2019 were $2.799 billion, or 35.5% of net sales, compared with $2.846 billion, or 37.1% of net sales which included impairment charges of approximately $193.2 million, during the comparable prior year period. Leverage was primarily driven by the impairment related to the sale of two businesses in the prior year (252 basis points), partially offset by increased domestic store payroll (72 basis points).
Net interest expense for thethirty-six weeks ended May 4, 2019 was $123.6 million compared with $120.2 million during the comparable prior year period. The increase was primarily due to an increase in average borrowing levels over the comparable prior year period.
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